The late-infantile-onset forms are the most genetically heterogeneous group among the autosomal recessively inherited neurodegenerative disorders, the neuronal ceroid lipofuscinoses (NCLs). The Turkish variant was initially considered to be a distinct genetic entity, with clinical presentation similar to that of other forms of late-infantile-onset NCL (LINCL), including age at onset from 2 to 7 years, epileptic seizures, psychomotor deterioration, myoclonus, loss of vision, and premature death. However, Turkish variant LINCL was recently found to be genetically heterogeneous, because mutations in two genes, CLN6 and CLN8, were identified to underlie the disease phenotype in a subset of patients. After a genomewide scan with single-nucleotide-polymorphism markers and homozygosity mapping in nine Turkish families and one Indian family, not linked to any of the known NCL loci, we mapped a novel variant LINCL locus to chromosome 4q28.1-q28.2 in five families. We identified six different mutations in the MFSD8 gene (previously denoted "MGC33302"), which encodes a novel polytopic 518-amino acid membrane protein that belongs to the major facilitator superfamily of transporter proteins. MFSD8 is expressed ubiquitously, with several alternatively spliced variants. Like the majority of the previously identified NCL proteins, MFSD8 localizes mainly to the lysosomal compartment. However, the function of MFSD8 remains to be elucidated. Analysis of the genome-scan data suggests the existence of at least three more genes in the remaining five families, further corroborating the great genetic heterogeneity of LINCLs.

Sialic acid storage diseases (SASD, MIM 269920) are autosomal recessive neurodegenerative disorders that may present as a severe infantile form (ISSD) or a slowly progressive adult form, which is prevalent in Finland (Salla disease). The main symptoms are hypotonia, cerebellar ataxia and mental retardation; visceromegaly and coarse features are also present in infantile cases. Progressive cerebellar atrophy and dysmyelination have been documented by magnetic resonance imaging (ref. 4). Enlarged lysosomes are seen on electron microscopic studies and patients excrete large amounts of free sialic acid in urine. A H+/anionic sugar symporter mechanism for sialic acid and glucuronic acid is impaired in lysosomal membranes from Salla and ISSD patients. The locus for Salla disease was assigned to a region of approximately 200 kb on chromosome 6q14-q15 in a linkage study using Finnish families. Salla disease and ISSD were further shown to be allelic disorders. A physical map with P1 and PAC clones was constructed to cover the 200-kb area flanked by the loci D6S280 and D6S1622, providing the basis for precise physical positioning of the gene. Here we describe a new gene, SLC17A5 (also known as AST), encoding a protein (sialin) with a predicted transport function that belongs to a family of anion/cation symporters (ACS). We found a homozygous SLC17A5 mutation (R39C) in five Finnish patients with Salla disease and six different SLC17A5 mutations in six ISSD patients of different ethnic origins. Our observations suggest that mutations in SLC17A5 are the primary cause of lysosomal sialic acid storage diseases.

Lysosomal free sialic acid-storage diseases include the allelic disorders Salla disease (SD) and infantile sialic acid-storage disease (ISSD). The defective gene, SLC17A5, coding for the lysosomal free sialic acid transporter was recently isolated by positional cloning. In the present study, we have identified a large number of mutations in SLC17A5 in patients presenting with either Salla disease or the ISSD phenotype. We also report for the first time the exon-intron boundaries of SLC17A5. All Finnish patients with SD (n=80) had a missense mutation changing a highly conserved arginine to cysteine (R39C); 91% of them were homozygotes for this old founder mutation. The compound-heterozygote patients, with the founder mutation in only one allele, presented with a more severe phenotype than did the homozygote patients. The same R39C mutation was also found both in most of the Swedish patients with SD and in a heterozygous form in five patients from central Europe who presented with an unusually severe (intermediate) SD phenotype. Ten different mutations, including deletions, insertions, and missense and nonsense mutations, were identified in patients with the most severe ISSD phenotype, most of whom were compound heterozygotes. Our results indicate some genotype-phenotype correlation in free sialic acid-storage diseases, suggesting that the phenotype associated with the homozygote R39C mutation is milder than that associated with other mutations.

Progressive myoclonus epilepsy of Unverricht-Lundborg type (EPM1) is an autosomal recessive neurodegenerative disorder caused by mutations in the cystatin B gene (CSTB) that encodes an inhibitor of several lysosomal cathepsins. An unstable expansion of a dodecamer repeat in the CSTB promoter accounts for the majority of EPM1 disease alleles worldwide. We here describe a novel PCR protocol for detection of the dodecamer repeat expansion. We describe two novel EPM1-associated mutations, c.149G4A leading to the p.G50E missense change and an intronic 18-bp deletion (c.168 þ 1_18del), which affects splicing of CSTB. The p.G50E mutation that affects the conserved QVVAG amino acid sequence critical for cathepsin binding fails to associate with lysosomes. This further supports the previously implicated physiological importance of the CSTB-lysosome association. Expression of CSTB mRNA and protein was markedly reduced in lymphoblastoid cells of the patients irrespective of the mutation type. Patients homozygous for the dodecamer expansion mutation showed 5-10% expression compared to controls. By combining database searches with RT-PCR we identified several alternatively spliced CSTB isoforms. One of these, CSTB2, was also present in mouse and was analyzed in more detail. In real-time PCR quantification, CSTB2 expression was less than 5% of total CSTB expression in all human adult and fetal tissues analyzed. In patients homozygous for the minisatellite mutation, the level of CSTB2 was reduced similarly to that of CSTB implicating regulation from the same promoter. The physiological significance of CSTB2 remains to be determined.

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